Composition of bis(fluorosulfonyl)imide lithium salt

ABSTRACT

A composition containing: at least 99.75% by weight of bis(fluorosulfonyl)imide lithium salt. A process for preparing the composition, including a) step of preconcentrating a composition C1 including an organic solvent OS1, water and bis(fluorosulfonyl)imide salt, to give a composition C2 including: the lithium salt of bis(fluorosulfonyl)imide in a content ranging from 35% to 50% relative to the total weight of composition C2; water in a mass content of less than or equal to 500 ppm relative to the total mass of composition C2; the preconcentration step being performed at a temperature of less than or equal to 50° C.; b) a step of concentrating composition C2; c) an optional step of crystallizing the composition obtained in step b).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Application No.17/059,856, filed on Nov. 30, 2020, which is a U.S. national stage ofInternational Application No. PCT/FR2019/051244, filed on May 28, 2019,which claims the benefit of French Application No. 1854788, filed onJun. 1, 2018. The entire contents of each of U.S. Application No.17/059,856, International Application No. PCT/FR2019/051244, and FrenchApplication No. 1854788 are hereby incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a composition based on the lithiumsalt of bis(fluorosulfonyl)imide.

TECHNICAL BACKGROUND

By virtue of their very low basicity, anions of sulfonylimide type areincreasingly used in the field of energy storage in the form ofinorganic salts in batteries, or of organic salts in supercapacitors orin the field of ionic liquids. Since the battery market is in fullexpansion and reduction of battery manufacturing costs has become amajor challenge, an inexpensive large-scale process for synthesizinganions of this type is necessary.

In the specific field of Li-ion batteries, the salt that is currentlythe most widely used is LiPF₆, but this salt has many drawbacks such aslimited thermal stability, sensitivity to hydrolysis and thus lowersafety of the battery. Recently, novel salts bearing the group FSO₂ ⁻have been studied and have demonstrated many advantages such as betterion conductivity and resistance to hydrolysis. One of these salts, LiFSI(LiN(FSO₂)₂), has shown highly advantageous properties which make it agood candidate for replacing LiPF₆.

The identification and quantification of impurities in salts and/orelectrolytes and the understanding of their impacts on batteryperformance have become paramount. For example, on account of theirinterference with electrochemical reactions, impurities bearing a labileproton led to reduced overall performance qualities and stability forLi-ion batteries. The application of Li-ion batteries makes it necessaryto have high-purity products (minimum amount of impurities).

There is a need for novel compositions based on lithiumbis(fluorosulfonyl)imide salt, for use thereof in batteries.

DESCRIPTION

The present invention relates to a composition comprising:

-   at least 99.75% by weight of the lithium salt of    bis(fluorosulfonyl)imide; and-   acetic acid in a mass content strictly greater than 0 and less than    or equal to 400 ppm. The mass contents mentioned above are relative    to the total weight of the composition.

In the context of the invention, the terms “lithium salt ofbis(fluorosulfonyl)imide”, “lithium bis(sulfonyl)imide”, “LiFSI”,“LiN(FSO₂)₂ ”, “lithium bis(sulfonyl)imide” and “lithiumbis(fluorosulfonyl)imide” are used equivalently.

In the context of the invention, the term “ppm” or “parts per million”is intended to mean ppm by weight.

Preferably, the abovementioned composition comprises at least 99.78%,preferentially at least 99.80%, advantageously at least 99.85% and moreadvantageously at least 99.90% by weight of lithium salt ofbis(fluorosulfonyl)imide relative to the total weight of saidcomposition. Preferably, the composition comprises at least 99.95%,preferentially at least 99.97%, advantageously at least 99.98% and moreadvantageously at least 99.99% by weight of lithium salt ofbis(fluorosulfonyl)imide relative to the total weight of saidcomposition.

According to one embodiment, the mass content of acetic acid in thecomposition is less than or equal to 350 ppm, preferentially less thanor equal to 300 ppm, advantageously less than or equal to 250 ppm, evenmore advantageously less than or equal to 200 ppm, for example less thanor equal to 150 ppm. Even more preferably, the content of acetic acid inthe composition is less than or equal to 100 ppm and in particular lessthan or equal to 50 ppm relative to the total weight of the composition.

According to one embodiment, the mass content of acetic acid in thecomposition is greater than or equal to 0.1 ppm, preferentially greaterthan or equal to 1 ppm, advantageously greater than or equal to 10 ppm,relative to the total weight of the composition.

According to one embodiment, the mass content of acetic acid in thecomposition ranges from 0.1 ppm to 300 ppm, preferably from 0.1 ppm to200 ppm, advantageously from 0.1 ppm to 150 ppm, even moreadvantageously from 0.1 ppm to 100 ppm relative to the total weight ofthe composition.

The abovementioned composition may also comprise:

-   a content of CI⁻ ions of less than or equal to 20 ppm by weight,    preferably less than or equal to 15 ppm, advantageously less than or    equal to 10 ppm by weight relative to the total weight of said    composition; and/or-   a content of F⁻ of less than or equal to 200 ppm, preferably less    than or equal to 50 ppm, advantageously less than or equal to 30 ppm    by weight relative to the total weight of said composition; and/or-   a content of H₂O of less than or equal to 200 ppm, preferably less    than or equal to 100 ppm, advantageously less than or equal to 50    ppm, even more advantageously less than or equal to 30 ppm by weight    relative to the total weight of said composition; and/or-   a content of SO₄ ²⁻ of less than or equal to 300 ppm, preferably    less than or equal to 200 ppm, advantageously less than or equal to    100 ppm, even more advantageously less than or equal to 50 ppm by    weight relative to the total weight of said composition; and/or-   a content of Na⁺ of less than or equal to 200 ppm, preferably less    than or equal to 100 ppm, advantageously less than or equal to 50    ppm, even more advantageously less than or equal to 20 ppm by weight    relative to the total weight of said composition; and/or-   a content of FSO₃Li of less than or equal to 500 ppm, preferably    less than or equal to 300 ppm, advantageously less than or equal to    200 ppm, even more advantageously less than or equal to 100 ppm and    in particular less than or equal to 20 ppm by weight relative to the    total weight of said composition; and/or-   a content of FSO₂NH₂ of less than or equal to 200 ppm, preferably    less than or equal to 100 ppm, advantageously less than or equal to    50 ppm, even more advantageously less than or equal to 20 ppm and in    particular less than or equal to 10 ppm by weight relative to the    total weight of said composition.

According to one embodiment, the composition comprises:

-   Cl⁻ ions in a content ranging from 0 to 20 ppm by weight, preferably    from 0 to 15 ppm and even more advantageously from 0 to 10 ppm by    weight relative to the total weight of said composition; and/or-   a content of F⁻ ranging from 0 to 200 ppm, preferably ranging from 0    to 50 ppm and advantageously ranging from 0 to 30 ppm by weight    relative to the total weight of said composition; and/or-   a content of H₂O ranging from 0 to 200 ppm, preferably ranging from    0 to 100 ppm, advantageously ranging from 0 to 50 ppm, even more    advantageously ranging from 0 to 30 ppm by weight relative to the    total weight of said composition; and/or-   a content of SO₄ ²⁻ ranging from 0 to 300 ppm, preferably ranging    from 0 to 200 ppm, advantageously ranging from 0 to 100 ppm, even    more advantageously ranging from 0 to 50 ppm by weight relative to    the total weight of said composition; and/or-   a content of Na⁺ ranging from 0 to 200 ppm, preferably ranging from    0 to 100 ppm, advantageously ranging from 0 to 50 ppm and even more    advantageously ranging from 0 to 20 ppm by weight relative to the    total weight of said composition; and/or-   a content of FSO₃Li ranging from 0 to 500 ppm, preferably ranging    from 0 to 300 ppm, advantageously ranging from 0 to 200 ppm, even    more advantageously ranging from 0 to 100 ppm and in particular    ranging from 0 to 20 ppm by weight relative to the total weight of    said composition; and/or-   a content of FSO₂NH₂ ranging from 0 to 200 ppm, preferably ranging    from 0 to 100 ppm, advantageously ranging from 0 to 50 ppm, even    more advantageously ranging from 0 to 20 ppm and in particular    ranging from 0 to 10 ppm by weight relative to the total weight of    said composition.

According to one embodiment, the composition comprises:

-   Cl⁻ ions in a content ranging from 0.1 to 20 ppm by weight,    preferably from 0.1 to 15 ppm and even more advantageously from 0.1    to 10 ppm by weight relative to the total weight of said    composition; and/or-   a content of F⁻ ranging from 0.1 to 200 ppm, preferably ranging from    0.1 to 50 ppm and advantageously ranging from 0.1 to 30 ppm by    weight relative to the total weight of said composition; and/or-   a content of H₂O ranging from 0.1 to 200 ppm, preferably ranging    from 0.1 to 100 ppm, advantageously ranging from 0.1 to 50 ppm, even    more advantageously from 0.1 to 30 ppm by weight relative to the    total weight of said composition; and/or-   a content of SO₄ ² _(¯) ranging from 0.1 to 300 ppm, preferably    ranging from 0.1 to 200 ppm, advantageously ranging from 0.1 to 100    ppm, even more advantageously ranging from 0.1 to 50 ppm by weight    relative to the total weight of said composition; and/or-   a content of Na⁺ ranging from 0.1 to 200 ppm, preferably ranging    from 0.1 to 100 ppm, advantageously ranging from 0.1 to 50 ppm and    even more advantageously ranging from 0.1 to 20 ppm by weight    relative to the total weight of said composition; and/or-   a content of FSO₃Li ranging from 0.1 to 500 ppm, preferably ranging    from 0.1 to 300 ppm, advantageously ranging from 0.1 to 200 ppm,    even more advantageously ranging from 0.1 to 100 ppm and in    particular ranging from 0.1 to 20 ppm by weight relative to the    total weight of said composition; and/or-   a content of FSO₂NH₂ ranging from 0.1 to 200 ppm, preferably ranging    from 0.1 to 100 ppm, advantageously ranging from 0.1 to 50 ppm, even    more advantageously ranging from 0.1 to 20 ppm and in particular    ranging from 0.1 to 10 ppm by weight relative to the total weight of    said composition.

The composition may also comprise a content of butyl acetate of lessthan or equal to 2000 ppm, preferably less than or equal to 1500 ppm,preferentially less than or equal to 1000 ppm, advantageously less thanor equal to 500 ppm, even more advantageously less than or equal to 250ppm, for example less than or equal to 150 ppm.

Preferably, the composition according to the invention is characterizedin that the sum of the total contents of acetic acid and of butylacetate is less than or equal to 2200 ppm, preferably less than or equalto 1700 ppm, advantageously less than or equal to 1200 ppm, relative tothe total weight of the composition. In particular, the composition issuch that: 0.1 ppm ≤ [acetic acid] + [butyl acetate] ≤ 1500 ppm, andpreferentially:

0.1 ppm ≤ [acetic acid] + [butyl acetate] ≤ 1000ppm.

The composition may also comprise a content of butanol of less than orequal to 500 ppm, preferably less than or equal to 300 ppm,preferentially less than or equal to 200 ppm, advantageously less thanor equal to 100 ppm, in particular less than or equal to 50 ppm,relative to the total weight of the composition.

The composition may also comprise a content of crystallization solvent,preferably chosen from chlorinated solvents and aromatic solvents, ofless than or equal to 1000 ppm, preferably less than or equal to 800ppm, preferentially less than or equal to 500 ppm, advantageously lessthan or equal to 200 ppm, in particular less than or equal to 100 ppmrelative to the total weight of the composition.

In the context of the invention, the term “crystallization solvent”means the solvent which may be used to crystallize the lithium salt ofbis(fluorosulfonyl)imide. This solvent is preferably dichloromethane ortoluene.

Preferably, the composition according to the invention is characterizedin that the sum of the total contents of acetic acid and of water isless than or equal to 400 ppm, preferably less than or equal to 300 ppm,advantageously less than or equal to 250 ppm, relative to the totalweight of the composition. In particular, the composition is such that:

0.1ppm ≤ [acetic acid] + [water] ≤ 150ppm,

and preferentially:

0.1ppm ≤ [acetic acid] + [water] ≤ 100ppm.

The amount of acetic acid and/or of butyl acetate and/or of butanoland/or of crystallization solvent is determined by proton NMR with aninternal standard: trifluorotoluene.

The composition according to the invention may be obtained via a processcomprising the following steps:

-   - a) step of preconcentrating a composition C1 comprising an organic    solvent OS1, water and bis(fluorosulfonyl)imide salt, to give a    composition C2 comprising:    -   o the lithium salt of bis(fluorosulfonyl)imide in a content        ranging from 35% to 50%, preferably from 40% to 45% by weight        relative to the total weight of composition C2;    -   o water in a mass content of less than or equal to 500 ppm,        preferably less than or equal to 300 ppm, advantageously less        than or equal to 100 ppm relative to the total mass of        composition C2;        -   said preconcentration step being performed at a temperature            of less than or equal to 50° C.;-   - b) a step of concentrating composition C2;-   - c) an optional step of crystallizing the composition obtained in    step b).

The lithium salt of bis(fluorosulfonyl)imide of composition C1 may beobtained via any known process for preparing said salt, for example asdescribed in WO 2015/158979 or WO 2009/1233328.

Composition C1 may be obtained via any known process for preparing thelithium salt of bis(fluorosulfonyl)imide.

Composition C1 may also be obtained via a process comprising thefollowing steps:

-   i) process for preparing the lithium salt of    bis(fluorosulfonyl)imide, said salt possibly being solid or in    solution in an organic solvent OS2;-   ii) step of placing in contact with an organic solvent OS2 in the    case where the salt LiFSI obtained in step i) is solid;-   iii) liquid-liquid extraction of said salt using the solution    containing the organic solvent OS2 and the salt, with deionized    water to form an aqueous solution of said salt of    bis(fluorosulfonyl)imide;-   iv) optional step of concentrating said aqueous solution;-   v) liquid-liquid extraction of the salt of bis(fluorosulfonyl)imide    using the aqueous solution with an organic solvent OS1, to recover    composition C1.

Preferably, composition C1 comprises:

-   a mass content of water ranging from 0.1% to 10%, preferentially    from 1% to 10%, advantageously from 1.5% to 10% by weight relative    to the total weight of said composition C1; and/or-   a mass content of the salt of bis(fluorosulfonyl)imide ranging from    5% to 30%, preferably from 5% to 20% by mass relative to the total    mass of the composition.

The abovementioned organic solvent OS2 may be chosen from the groupconstituted of esters, nitriles, ethers, chlorinated solvents andaromatic solvents, and mixtures thereof. Preferably, the solvent OS2 ischosen from dichloromethane, ethyl acetate, butyl acetate,tetrahydrofuran, acetonitrile and diethyl ether, and mixtures thereof.Preferably, the organic solvent OS2 is butyl acetate.

According to the invention, the abovementioned step iii) may be repeatedat least once.

According to one embodiment, the organic solvent OS1 is chosen from thegroup constituted of esters, nitriles, ethers, chlorinated solvents andaromatic solvents, and mixtures thereof. Preferably, the solvent OS1 ischosen from ethers and esters, and mixtures thereof. For example,mention may be made of methyl t-butyl ether, cyclopentyl methyl ether,ethyl acetate, propyl acetate, butyl acetate, dichloromethane,tetrahydrofuran, acetonitrile and diethyl ether, and mixtures thereof.Preferably, the solvent OS1 is chosen from methyl t-butyl ether,cyclopentyl methyl ether, ethyl acetate, propyl acetate and butylacetate, and mixtures thereof, the organic solvent OS2 preferentiallybeing butyl acetate.

The preconcentration step a) is preferably performed at a temperatureranging from 25° C. to 45° C., preferably from 30° C. to 40° C.

Preferably, the preconcentration step a) is performed under reducedpressure, for example at a pressure of less than or equal to 50 mbarabs, in particular at a pressure of less than or equal to 30 mbar abs.

The preconcentration step a) may be performed by any concentratingmeans, for example using an evaporator.

Preferably, the abovementioned step b) is performed in a short-paththin-film evaporator, under the following conditions:

-   temperature of between 30° C. and 95° C., preferably between 30° C.    and 90° C., preferentially between 40° C. and 85° C., in particular    between 60° C. and 80° C.,-   pressure of between 10⁻³ mbar abs and 5 mbar abs, in particular    between 5×10_(¯) ¹ and 2 mbar abs,-   residence time of less than or equal to 5 min, preferably less than    or equal to 3 min.

In the context of the invention, and unless otherwise mentioned, theterm “residence time” means the time which elapses between the entry ofthe solution of lithium bis(fluorosulfonyl)imide salt (in particularobtained on conclusion of the abovementioned step b)) into theevaporator and the exit of the first drop of the solution.

According to a preferred embodiment, the temperature of the condenser ofthe short-path thin-film evaporator is between -50° C. and 5° C.,preferably between -35° C. and 5° C. In particular, the condensertemperature is -5° C.

The short-path thin-film evaporators according to the invention are alsoknown as “wiped-film short-path” (WFSP) evaporators. They are typicallyreferred to as such since the vapors generated during the evaporationcover a short path (travel a short distance) before being condensed inthe condenser.

Among the short-path thin-film evaporators, mention may notably be madeof the evaporators sold by the companies Buss SMS Ganzler ex Luwa AG,UIC GmbH or VTA Process.

Typically, the short-path thin-film evaporators may comprise a condenserfor the solvent vapors placed inside the machine itself (in particularat the center of the machine), unlike other types of thin-filmevaporator (which are not short-path evaporators) in which the condenseris outside the machine.

In this type of machine, the formation of a thin film, of product to bedistilled, on the hot inner wall of the evaporator may typically beensured by continuous spreading over the evaporation surface with theaid of mechanical means specified below.

The evaporator may notably be equipped, at its center, with an axialrotor on which are mounted the mechanical means that allow the formationof the film on the wall. They may be rotors equipped with fixed vanes,lobed rotors with three or four vanes made of flexible or rigidmaterials, distributed over the entire height of the rotor, or rotorsequipped with mobile vanes, paddles, doctor blades or guided scrapers.In this case, the rotor may be constituted by a succession ofpivot-articulated paddles mounted on a shaft or axle by means of radialsupports. Other rotors may be equipped with mobile rollers mounted onsecondary axles and said rollers are held tight against the wall bycentrifugation. The spin speed of the rotor, which depends on the sizeof the machine, may be readily determined by a person skilled in theart. The various spindles may be made of various materials: metallic,for example steel, steel alloy (stainless steel), aluminum, orpolymeric, for example polytetrafluoroethylene PTFE, or glass materials(enamel); metallic materials coated with polymeric materials.

According to one embodiment, the solution is introduced into theshort-path thin-film evaporator with a flow rate of between 700 g/h and1200 g/h, preferably between 900 g/h and 1100 g/h for an evaporationarea of 0.04 m².

According to one embodiment, the abovementioned process also comprises astep c) of crystallization of the lithium bis(fluorosulfonyl)imide saltobtained on conclusion of the abovementioned step b).

The crystallization step may be performed in an organic solvent(“crystallization solvent”) chosen from chlorinated solvents, forinstance dichloromethane, and aromatic solvents, for instance toluene.

Preferably, the LiFSI composition obtained on conclusion of step c) isrecovered by filtration.

Preferably, the crystallization is performed at a temperature of lessthan or equal to 25° C., preferentially less than or equal to 15° C.

The solvents of ester type used for preparing the lithium salt ofbis(fluorosulfonyl)imide may be hydrolyzed (in the presence of water) todecomposition products: acid and alcohol. Butyl acetate may notably behydrolyzed to acetic acid and butanol. The inventors have discoveredthat a high content of acetic acid may harm the performance of thebattery. Thus, the process according to the invention advantageouslymakes it possible to reduce, or even to prevent, the partialdecomposition of the organic solvents used, for instance butyl acetateto acetic acid and/or butanol.

The composition according to the invention advantageously gives improvedperformance in batteries. In particular, the composition according tothe invention has at least one of the following advantages:

-   the corrosion of the aluminum current collector is advantageously    reduced and/or zero;-   improved service life of the battery;-   improved battery performance.

The present invention also relates to the use of the compositionaccording to the invention in batteries, notably in Li-ion batteries.

In particular, the composition according to the invention may be used inLi-ion batteries of mobile devices (for example cell phones, cameras,tablets or laptop computers), or electric vehicles, or for storingrenewable energy (such as photovoltaic or wind energy).

In the context of the invention, the term “between x and y” or “rangingfrom x to y” means a range in which the limits x and y are included. Forexample, the temperature “between 30 and 100° C.” notably includes thevalues 30° C. and 100° C.

All the embodiments described above may be combined with each other.

The present invention is illustrated by the example which follows, towhich it is not, however, limited.

EXPERIMENTAL SECTION Content of Residual Solvents: Headspace Method

-   Equipment:Agilent 6890-   Chromatographic headspace system: Agilent 6890-   HP-5 column length: 30 m, inside diameter 0.32 mm, active phase    thickness: 0.25 µm Chromatographic conditions: oven at 60° C. for 2    minutes then ramp of 30° C./minute up to 300° C. and then    maintenance at 300° C. for 2 minutes.-   Injector: 250° C.-   FID detector at 300° C.-   Headspace conditions: 80° C. for 30 minutes-   Sampling: 0.05 g of LiFSI dissolved in 200 ml of aqueous dimethyl    sulfoxide DMSO solution: DMSO/ultra-pure water: 20/80 by volume. 2    ml of aqueous NaCl solution (20% by mass) are then added. The    solution obtained is then transferred into a vial, which is sealed.

Quantification:

-   Calibration was performed using pure products. The detection limits    were evaluated:-   Butyl acetate = 0.01% by weight-   1-Butanol = 0.01% by weight-   Dichloromethane = 0.01% by weight-   Toluene = 0.05% by weight-   Acetic acid = 5% by weight-   The acetic acid detection limit is particularly high.

Content of Residual Solvents NMR Method

The 1H NMR analysis conditions are as follows:

Equipment: The NMR spectra and quantifications were performed on aBrüker AV 400 spectrometer, at t 376.47 MHz for ¹⁹F, on a 5 mm probe ofBBFO⁺ type.

Sampling:

The LiFSI samples are dissolved in DMSO-d6 (about 30 mg in 0.6 ml).

Quantification:

The absolute quantification in ¹⁹F NMR and proton NMR is performed bydosed addition of α,α,α-trifluorotoluene (TFT, Aldrich) to the tubecontaining the sample. The signals for the fluorinated species to beassayed are integrated in comparison with that of the CF₃ of thisinternal standard, according to the method that is well known to thoseskilled in the art. In proton NMR, the quantification is performed in asimilar manner relative to the signal for the aromatic protons of thetrifluorotoluene. The quantification limit of a species is of the orderof a 50th of a ppm.

Example 1

A solution of 134 g of LiFSI in 823 g of butyl acetate (which may beobtained, for example, according to the process described in WO2015/158979). The LiFSI concentration is approximately 10% by weight andthe water content of this solution is 3% by weight. The water content ofthis solution is higher than the solubility of water in butyl acetatedue to the association of the lithium salt with water. A firstconcentration by evaporation of the solvent is performed with a rotaryevaporator at 40° C. under reduced pressure (P < 30 mbar). A solutionwith a solids content of 42% and a water content, measured by titration,of 430 ppm by weight is obtained. The final concentration is performedin a WFSP (wiped-film short-path) evaporation machine at a temperatureof 80° C. under a vacuum of 0.5 mbar. This concentrate is taken up indichloromethane. The LiFSI crystallizes rapidly. After a contact time of1 hour, solid LiFSI is obtained and is recovered by filtration and driedunder vacuum for at least 24 hours. The mass of solid LiFSI is 110 g,i.e., a yield of 82%.

The analysis of the residual solvents in the LiFSI obtained is asfollows:

Weight% Headspace method NMR method Butyl acetate 0.12 0.15Dichloromethane 0.07 0.07 Acetic acid Not detected Not detected ButanolNot detected Not detected

Example 2 (comparative)

A solution of 53 g of LiFSI in 640 g of butyl acetate (obtained, forexample, according to the process described in WO 2015/158979). Thewater content is 3.2% by weight. The solution is evaporated under vacuumat 70° C. A solution with a solids content of 40% and a water content of1050 ppm by weight is obtained. The final concentration is performed ina WFSP (wiped-film short-path) evaporation machine at a temperature of80° C. under a vacuum of 0.5 mbar.

The concentrate is taken up in dichloromethane. The LiFSI crystallizesrapidly. After a contact time of 1 hour, 44 g of LiFSI are obtained andare recovered by filtration and dried under vacuum for at least 24hours.

The residual solvent analysis is given below:

Weight% Headspace NMR Butyl acetate 0.15 0.16 Dichloromethane 0.07 0.07Acetic acid Not detected 0.055 Butanol Not detected Not detected

The headspace measurement method by gas chromatography introduces a biasin the quantification of the organic species since the measurement isdirectly linked to the liquid/vapor equilibria of the system(underestimated results). The NMR assay method is more reliable since itis a direct measurement of the composition, and it has a lower detectionlimit than the headspace method.

Example 3: Chronoamperometry and Cyclic Voltammetry Tests

The electrolyte solutions No. 1 and No. 2 are prepared by dissolving theLiFSI prepared according to the preceding Examples 1 and 2 in a 3/7 byvolume ethylene carbonate/ethyl methyl carbonate mixture. The LiFSIconcentration is 0.8 mol/l. Furthermore, 2% by weight of fluoroethylenecarbonate is added to each electrolyte.

Cyclic Voltammetry Test

The cyclic voltammetry tests are performed on button cells with alithium metal anode and an aluminum cathode with the preparedelectrolyte. The voltage is varied between 0 and 6 V with a sweep speedof 1 mV/s over three cycles. The current obtained on the third cycle,thus after the possible formation of the passivation layer, is noted.

The table below presents the results:

Measurement voltage Electrolyte No. 1 (with the LiFSI obtained inExample 1) Electrolyte No. 2 (with the LiFSI obtained in Example 2) 4.2V 29 µA 154 µA 4.5 V 1034 µA 1948 µA 5V 543 µA 933 µA

It is observed that electrolyte No. 2 leads to a current having a higherintensity than that obtained with electrolyte No. 1. The current with ahigher intensity (electrolyte No. 2) indicates greater corrosion of thealuminum.

Chronoamperometry:

This test consists in imposing a constant voltage to a battery of thesame type as that described for the cyclic voltammetry test and inmonitoring the current intensity across the cell. The objective is tomeasure the leakage current, residual current of constant intensity,which reflects the polarization of the battery and thus its servicelife. The greater the leakage current, the shorter will be the servicelife of the battery. The test was performed at 4 V.

For the cell prepared with the LiFSI of Example 1, the leakage currentis 3.1 µA. The leakage current for the cell manufactured with the LiFSIof Example 2 is 15 µA.

These two tests show that electrolyte No. 2 prepared with an LiFSIcontaining 550 ppm of acetic acid has degraded performance relative toelectrolyte No. 1 prepared with the LiFSI of Example 1.

1. A composition comprising: at least 99.75% by weight of a lithium saltof bis(fluorosulfonyl)imide; and at least one of: butyl acetate in amass content ranging from greater than 0 to less than or equal to 2000ppm, Cl⁻ ions in a mass content ranging from greater than 0 to less thanor equal to 20 ppm, or SO₄ ²⁻ ions in a mass content ranging fromgreater than 0 to less than or equal to 300 ppm.
 2. The composition asclaimed in claim 1, comprising at least 99.78% by weight of the lithiumsalt of bis(fluorosulfonyl)imide relative to the total weight of saidcomposition.
 3. The composition as claimed in claim 1, comprising atleast 99.95% by weight of the lithium salt of bis(fluorosulfonyl)imiderelative to the total weight of said composition.
 4. The composition asclaimed in claim 1, further comprising a content of F⁻ ions ranging from0 to 200 ppm relative to the total weight of said composition.
 5. Thecomposition as claimed in claim 1, further comprising a content of H₂Oranging from 0 to 200 ppm by weight relative to the total weight of saidcomposition.
 6. The composition as claimed in claim 1, furthercomprising a content of Na⁺ ions ranging from 0 to 200 ppm by weightrelative to the total weight of said composition.
 7. The composition asclaimed in claim 1, further comprising a content of FSO₃Li ranging from0 to 500 ppm by weight relative to the total weight of said composition.8. The composition as claimed in claim 1, further comprising a contentof FSO₂NH₂ ranging from 0 to 200 ppm by weight relative to the totalweight of said composition.
 9. The composition as claimed in claim 1,comprising a content of butanol of less than or equal to 500 ppmrelative to the total weight of the composition.
 10. The composition asclaimed in claim 1, comprising a content of crystallization solvent ofless than or equal to 1000 ppm relative to the total weight of thecomposition.
 11. The composition as claimed in claim 1, wherein thecontent of water is less than or equal to 400 ppm relative to the totalweight of the composition.
 12. The composition as claimed in claim 1,wherein the composition is prepared from a process comprising apre-concentration step being performed at a temperature of less than orequal to 50° C.
 13. The composition as claimed in claim 1, furthercomprising a crystallization solvent.
 14. The composition as claimed inclaim 1, comprising Cl⁻ ions in a mass content ranging from greater than0 to less than or equal to 20 ppm.
 15. The composition as claimed inclaim 1, comprising SO₄ ²⁻ ions in a mass content ranging from greaterthan 0 to less than or equal to 300 ppm.
 16. A process for preparing acomposition as claimed in claim 1, comprising the following steps: a)preconcentrating a composition C1 comprising an organic solvent OS1,water and bis(fluorosulfonyl)imide salt, to give a composition C2comprising: the lithium salt of bis(fluorosulfonyl)imide in a contentranging from 35% to 50% relative to the total weight of composition C2;water in a mass content of less than or equal to 500 ppm relative to thetotal mass of composition C2; said preconcentration step being performedat a temperature of less than or equal to 50° C.; b) concentratingcomposition C2; c) optionally crystallizing the composition obtained instep b).
 17. The process as claimed in claim 16, wherein composition C1comprises: a mass content of water ranging from 0.1% to 10% relative tothe total weight of said composition C1; and/or a mass content of thelithium salt of bis(fluorosulfonyl)imide ranging from 5% to 30% by massrelative to the total mass of the composition.
 18. The process asclaimed in claim 16, wherein the organic solvent OS1 is chosen from thegroup consisting of esters, nitriles, ethers, chlorinated solvents,aromatic solvents, and mixtures thereof.
 19. The process as claimed inclaim 16, in which the preconcentration step a) is performed underreduced pressure.
 20. The process as claimed in claim 16, in which stepb) is performed in a short-path thin-film evaporator, under thefollowing conditions: a temperature of between 30° C. and 95° C., apressure of between 10⁻³ mbar abs and 5 mbar abs, a residence time ofless than or equal to 5 min.